1. Field of the Invention
The present invention relates to a magnetic record erase circuit and, in particular, to a magnetic record erase circuit which is capable of substantially erasing frequency signals such as video signals recorded in a rotary magnetic recording medium such as a magnetic disc, a magnetic drum or the like.
2. Description of the Prior Art
Recently, there has been developed an electronic still camera system in which an image pickup device such as a solid image pickup element or an image pickup tube is combined with a recording device using as an inexpensive magnetic recording medium having a relatively large memory capacity such as a magnetic disc or the like, whereby an object can be still photographed electronically and the still image of the object is then recorded onto the rotary magnetic recording medium, and such still image can be reproduced by means of a printer, a television system, or the like separately arranged. Also, there has been proposed a video magnetic recording system in which still images recorded in a visual recording medium such as an ordinary film, a photographic paper or the like can be photographed and then recorded onto a magnetic recording medium of the above-mentioned type.
However, in the above-mentioned electronic still camera and video magnetic recording systems, there arises a requirement for one recording unit of the magnetic recording medium, for example, a video signal once recorded in a track to be erased with the video signal being recorded again in the same track. This requirement, in most cases, is due to recording failures which are not related to the characteristics of the magnetic recording medium. For example, in an case of the electronic still camera, such recording failures include "out of errors", errors in releasing the shutter of the camera and the like; and, in the video magnetic recording system, the requirement may be caused not only by bad adjustments in the white balance, color correction, density correction or the like of a video signal recorded therein but also by editing of frames recorded in respective tracks.
Also, in order to record a video signal representing a new still image onto a magnetic disc which stores old still images, it is necessary to erase all of the video signals recorded in the magnetic disc so that rerecording can be performed satisfactorily.
When erasing a given track selectively, video signals in the given track may be substantially erased, for example, by applying erase signals having a higher frequency than the band of the video signals.
In one of the conventional erase methods, there is employed a band ranging from 0 MHz to 12 MHz. Specifically, a band of 2.5 MHz or lower is used for line sequential color difference signals and a band of higher than 2.5 MHz is used for brightness signals. In this method, 1 field of video signals are recorded into 1 track using a frequency modulation technique. When erasing a track in which the video signals having such frequency spectra are recorded, the video signals in the track may be erased substantially by applying, for example, erase signals having a frequency on the order of 10 MHz for a given period of time. Also, erase signals having a plurality of frequencies may be used. For example, first, an erase signal on the order of 10 MHz may be applied for a given period of time and subsequently another erase signal on the order of 20 MHz may be applied for a given period of time. However, from the results of experiments by the inventors, in either of the above-mentioned conventional erasing methods, the video signals recorded in a given track can not be erased effectively, that is, to such a degree that satisfactory rerecording is possible.
Further, in a method disclosed in Japanese patent application No. 60-45748, the discrete frequency values described above are not used, but frequencies are varied successively from lower frequencies to higher frequencies so as to carry out such an erasing operation. In addition, there is known a direct current erasing method in which an erasing direct current gradually decreasing in level in synchronization with a 1 V (that is, 1 field) period of a recorded video signal recorded is applied to a magnetic head. However, as well known, since a signal having a lower frequency component will magnetize a magnetic recording material layer down to the comparatively deeper position thereof, in order to erase the magnetized recording material layer to a satisfactory degree, it is necessary to apply an erase signal having a lower frequency at sufficiently long frequency sweep cycles. As a result of this, in order to erase the recorded video signals effectively, a long time is required. For example, a fairly long time, e.g., about 50 seconds is necessary to erase the video signals recorded in all of 50 tracks provided in a video disc having a diameter of 47 mm.